An automobile is provided with power transmission device for transmitting engine power to front or rear wheels.
Generally, the power transmission device consists of a clutch, a gearbox, a differential gear, etc. The clutch couples two working parts in such a way as to permit connection or disconnection at will without the necessity of bringing both parts to rest. The gearbox is the complete system of gear wheels for changing the speed from that of an input shaft to that of an output shaft, or changing the direction of rotation of a shaft with or without a speed change. The differential gear is an arrangement of gears that allows one driving wheel of a car to turn faster than the other when the car goes around a corner.
Various power transmission devices have been designed, and some of them have been disclosed in patent gazette.
A power transmission device with multiple overdrive is disclosed in U.S. Ser. No. 09/476,616 by the applicant of this application. A gearbox comprises a plurality of speed change gears provided with a carrier having a specific shape. Accordingly as a carrier of one of the speed change gears is selectively engaged so as to select a desired speed and direction of output torque, the power transmission device accomplishes speed change operation effectively. Also, the power transmission device can be automatically operated, and the configuration thereof is simple and compact.
However, it is difficult to control wheel speed because the configuration between the output shaft on which speed change gears are coaxially fixed and the wheels is complicated. That is, if normal final reduction gear and differential gear is used, it is difficult to control wheel speed, because the configuration of this power transmission device is different from that of a conventional power transmission device.
Additionally, a conventional four-wheel drive for transmitting power to both front and rear wheels is complicated in the structural aspect, and cannot accomplish multiple-stage speed change operation effectively. That is, the four-wheel drive according to the prior art is installed at one portion of a speed change device, and a device to selectively connect the speed change device to the four-wheel drive is complicated in the structural aspect, thus it is impossible to change an output speed in multiple-stage.